first day on the new job and i went with the service manager to remove hotshot from a medium temp system and retrofit 416A in it's place. all went smooth and we started with 14 lbs of 416A which is same amount of hotshot we removed. started up system and pressures seemed ok but superheat was really low (3-4 degrees) system has 3 seperate TXV's. we let it run that way long enough to bring box temp close to setpoint and then checked sight glass again.. nearly empty. added 2 lbs more since the system has a 20+ lb receiver and rechecked sight glas.. same as before and super heat unchanged.. went in and checked super heat at each seperate TXV and attempted to adjust for correct operation..heres where it gets weird.. the TXv's would either be wide open or shut.. no modulation in between. one moment my suction line leaving the evap would be 39.0 degrees and then next it was dropping like a rock to 23 degrees. all three of the txv's acted like this and adjusting made no difference. box temp was 39.0 the txv's are alco and are 25+ years old. suction pressure was 16 when box was at 37 degrees and head pressure held steady at 100 with headmaster doing it's job.*Note when the TXV's were not feeding the sight glass would nearly clear up.. this is only the second time i have worked with 416A and i had the same issue before. wondering if theres a trick to it that we werent aware of.